Scan path interface with circular shift register, logic, register sets

ABSTRACT

The disclosure describes a novel method and apparatus for allowing response data output from the scan outputs of a circuit under test to be formatted and applied as stimulus data input to the scan inputs of the circuit under test. Also the disclosure described a novel method and apparatus for allowing the response data output from the scan outputs of a circuit under test to be formatted and used as expected data to compare against the response data output from the circuit under test. Additional embodiments are also provided and described in the disclosure.

CROSS REFERENCE TO RELATED PATENTS

This application is a divisional of application Ser. No. 15/400,239,filed Jan. 6, 2017, now U.S. Pat. No. 9,746,518, issued Aug. 29, 2017;

Which was a divisional of application Ser. No. 15/259,619, filed Sep. 8,2016, now U.S. Pat. No. 9,575,122, granted Feb. 21, 2017;

Which was a divisional of application Ser. No. 15/089,978, filed Apr. 4,2016, now U.S. Pat. No. 9,470,758, granted Oct. 18, 2016;

Which was a divisional of application Ser. No. 14/735,806, filed Jun.10, 2015, now U.S. Pat. No. 9,329,232, granted May 3, 2016;

Which was a divisional of application Ser. No. 13/890,781, filed May 9,2013, now U.S. Pat. No. 9,081,059, granted Jul. 14, 2015;

Which was a divisional of application Ser. No. 13/568,533, filed Aug. 7,2012, now U.S. Pat. No. 8,493,992, granted Jul. 23, 2013;

Which was a divisional of application Ser. No. 13/325,740, filed Dec.14, 2011, now U.S. Pat. No. 8,271,840, granted Sep. 18, 2012;

Which was a divisional of application Ser. No. 12/607,436, filed Oct.28, 2009, now U.S. Pat. No. 8,099,642, granted Jan. 17, 2012;

Which claims priority from Provisional Application No. 61/110,804, filedNov. 3, 2008.

This application is related to application Ser. No. 08/931,791, filedSep. 16, 1997, now U.S. Pat. No. 6,260,165, granted Jul. 10, 2001.

FIELD OF THE DISCLOSURE

This disclosure relates generally to device scan testing and inparticular to device scan testing whereby the scan response outputs froma device under test are used for testing the device under test.

BACKGROUND OF THE DISCLOSURE

Most electrical circuits today, which may be IC devices or embeddedsub-circuits within IC devices, are tested using parallel scan pathapproaches whereby the parallel scan paths are used to input and applytest stimulus patterns to a circuit's combinational logic and to captureand output test response patterns from the circuit's combinationallogic.

FIG. 1 illustrates a conventional example of a circuit's 100combinational logic 102 being tested via parallel scan paths 1-n 104. Aspreviously mentioned, circuit 100 could be an IC or an embeddedsub-circuit within an IC. Each scan path 1-n has a scan input (SI-1-n)106 and a scan output (SO-1-n). Each scan path 1-n is also coupled to ascan enable (SEN) input 110 and to a scan clock (SCK) input 112. The SENand SCK inputs cause the scan paths 1-n to; (1) input stimulus bitstreams 122 during each stimulus input shift cycle 114 to be applied asstimulus 116 to the combinational logic, (2) capture the resultingresponse 118 from the combinational logic, and (3) shift the capturedresponse data out as response bit streams 124 during each responseoutput shift cycle 120. The structure and operations of the parallelscan paths of FIG. 1 are well known.

FIG. 2 illustrates a typical arrangement between a scan tester 200 and acircuit 100 within an IC 200 that is to be scan tested. Typical scantesters comprise a stimulus data memory 204 for storing stimuluspatterns 114 to be input to circuit 100 via SI-1-n, an expected datamemory 206 for storing the expected data (ED-1-n) patterns from circuit100, a compare pass/fail circuit 208 for comparing the expected datafrom expected data memory 206 against the response patterns 120 outputfrom circuit 100 via SO-1-n, and a controller circuit 210 to control theoperation of the stimulus data memory, response data memory, comparepass/fail circuit, and the SCK and SEN inputs to circuit 100 via bus212.

During test, the controller circuit 210 operates the SCK and SEN inputsto circuit 100 to shift in stimulus bit streams 122 during each shiftcycle 114 via SI-1-n, capture response data 118 from combinational logic102 of circuit 100, and shift out response bit streams 124 during eachshift cycle 120 via SO-1-n. Each bit in the response bit stream 124 iscompared to an expected data bit from the expected data memory 206. Thecompare pass/fail circuit is controlled by the SCK and SEN signals toallow it to operate in synchronicity with circuit 100 to allow it toknow when to compare the response bits against the expected data bits.The compare pass/fail circuit 208 may be simple or complex. A simplecompare pass/fail circuit 208 may simply detect and log the firstmismatch between the response bits output from circuit 100 and theexpected data bits output from the expected data memory 206. However, acomplex compare pass/fail circuit 208 may detect and log all mismatchesbetween the response bit outputs from circuit 100 and the expected databits output from the expected data memory 206. Further, the more complexcompare pass/fail circuit 208 may include the ability to mask offcertain compare operations between response bits and expected data bitsduring the test. At the end of test, the controller circuit 210 accessesthe compare pass/fail circuit, via bus 212, to obtain pass/failinformation.

The stimulus data memory 204 and expected data memory 206 of tester 200may need to be very large. For example, circuit 100 may contain 32 scanpaths 104 each being 50,000 bits long. In this example, the stimulusdata memory 204 needs to store 50,0000 32-bit wide stimulus bit patternsto be input to circuit 100 during each shift cycle 114 operation, andthe expected data memory 206 needs to store 50,0000 32-bit wide expecteddata bit patterns to match against the response bit patterns output fromcircuit 100 during each shift cycle operation. Assuming 10,000 shiftcycle operations are required for the test, the memories 204 and 206would each need to be able to store 500 million 32-bit patterns.Additionally, memories 204 and 206 would need to be fast memories toallow operating the shift operations at high SCK rates to reduce theshift cycle times during test. The need for large high speed stimulusand expected data memories increases the cost of scan testers 200, whichis reflected in the selling cost of the IC to the customer.

BRIEF SUMMARY OF THE DISCLOSURE

This disclosure describes a method and apparatus for allowing theresponse data patterns from parallel scan paths of a circuit to be usedto provide stimulus data patterns for input to the parallel scan pathsof the circuit. Additionally, the response data patterns can provideexpected data patterns for comparing against the response data patterns.

BRIEF DESCRIPTION OF THE VIEWS OF THE DRAWINGS

FIG. 1 illustrates a conventional scan test architecture within acircuit.

FIG. 2 illustrates a conventional scan test arrangement between acircuit under test and a tester.

FIG. 3 illustrates a scan test arrangement between a circuit under testand a tester according to the disclosure.

FIG. 3A illustrates the circuit under test of FIG. 3 in more detailaccording to the disclosure.

FIG. 4 illustrates a formatter circuit for reusing response data from acircuit to supply stimulus data to the circuit according to thedisclosure.

FIGS. 5A-5D illustrate individual sections of the formatter circuit ofFIG. 4 according to the disclosure.

FIG. 6A illustrates using a multiplexer to realize a formatter sectionof FIG. 5 according to the disclosure.

FIG. 6B illustrates the timing of inputting control to the multiplexerof FIG. 6A to produce stimulus data from response data according to thedisclosure.

FIG. 6C illustrates the conventional timing of inputting stimulus datato a circuit from a tester.

FIG. 7 illustrates a stimulus data formatter with independentlycontrolled multiplexer sections according to the disclosure.

FIG. 8 illustrates the formatter of FIG. 7 being controlled by a writestrobe sequencer circuit according to the disclosure.

FIG. 8A illustrates an example implementation of the write strobesequencer circuit of FIG. 8 according to the disclosure.

FIG. 9 illustrates the formatter of FIG. 7 being controlled by anaddressable write strobe circuit according to the disclosure.

FIG. 9A illustrates an example implementation of the addressable writestrobe circuit of FIG. 9 according to the disclosure.

FIG. 10 illustrates a stimulus data formatter with commonly controlledmultiplexer sections according to the disclosure.

FIG. 11 illustrates a circuits stimulus input pattern set and responseoutput patterns set generated by an automatic test pattern generationtool.

FIGS. 12A and 12B illustrate the process of producing stimulus inputsfrom response outputs using common control bus timings according to thedisclosure.

FIG. 13 illustrates an arrangement between a tester and a circuit undertest using any one of the formatter control interfaces of FIGS. 7, 8, 9and 10 according to the disclosure.

FIG. 14 illustrates the use of a formatter circuit in combination with acompactor circuit according to the disclosure.

FIG. 15 illustrates the use of a formatter circuit in combination with aMISR circuit according to the disclosure.

FIG. 16 illustrates a device containing a first formatter for providingstimulus data and a second formatter for providing expected dataaccording to the disclosure.

FIG. 17 illustrates a formatter circuit for reusing response data from acircuit to supply expected data according to the disclosure.

FIGS. 18A-18D illustrate individual sections of the formatter circuit ofFIG. 17 according to the disclosure.

FIG. 19 illustrates an expected data formatter with independentlycontrolled multiplexer sections according to the disclosure.

FIG. 20 illustrates an expected data formatter with commonly controlledmultiplexer sections according to the disclosure.

FIGS. 21A and 21B illustrate the process of producing expected data fromresponse outputs using common control bus timings according to thedisclosure.

FIG. 22 illustrates a circuits scan test architecture.

FIG. 23 illustrates wrapping the scan inputs and scan outputs of theFIG. 22 circuit with formatter and compare pass/fail circuits accordingto the disclosure.

FIG. 24 illustrate placing enable circuitry on the test interfacesignals of the FIG. 23 circuit arrangement according to the disclosure.

FIG. 25 illustrates a device with multiple FIG. 24 circuits to be testedusing a common test interface according to the disclosure.

FIG. 26 illustrates a device self-test architecture using stimulus andresponse formatter circuits and a JTAG interface.

FIG. 27 illustrates a device self-test architecture using a stimulusformatter circuit, a MISR circuit and a JTAG interface.

DETAILED DESCRIPTION OF THE DISCLOSURE

FIG. 3 illustrates an IC 304 containing a first embodiment of thedisclosure coupled to a scan tester 300. The IC comprises an embeddedcircuit 301 to be scan tested and a formatter circuit 306. Circuit 301is similar to circuit 100 of FIGS. 1 and 2 in that it has scan inputs(SI-1-n) 310, scan outputs (SO-1-n) 108, a SEN control input 110 and aSCK 112 control input. Circuit 301 differs from circuit 100 in that itincludes a serial interface bus 307 consisting of a serial input (SI),serial mode input (SM), and a serial output (SO). The serial interfacebus 307 of circuit 301 is controlled by bus 212 of tester 300. Circuit301 receives SCK and SEN control inputs from the tester via bus 212 andoutputs response data to the tester via SO-1-n 108 as previouslydescribed in regard to FIG. 2.

The formatter circuit 306, according to the disclosure, is used toprovide the stimulus data patterns to circuit 301 via the SI-1-n inputs310. The formatter 306 inputs response data patterns from the SO-1-noutputs 108 of circuit 301, inputs stimulus data control signals 308(CTL-(1-n)) from tester 300, and outputs stimulus data patterns to theSI-1-n inputs 310 of circuit 301. The tester 300 is the similar totester 200 of FIG. 2 with the exception that it uses a control memory302 instead of a stimulus data memory 204 and it provides the serialinterface bus 307 to circuit 301.

FIG. 3A illustrates the circuit 301 in more detail. As seen, circuit 301contains a circuit 100 to be scan tested and multiplexers 303 forallowing the scan inputs 106 of the circuit 100 scan paths 104 to comefrom either the formatter 306 or from the serial input (SI) of serialinterface bus 307. When the serial mode (SM) signal of interface 307 isset to a first logic level, the scan paths 104 are all placed in series,via the multiplexers 303, to allow the scan paths 104 to be scanned viathe serial input (SI) and serial output (SO) of interface 307. While inserial mode, the SCK and SEN control signals operate the scan paths 104to shift data and to capture data as previously described. The serialinterface bus 307 is used at the beginning of a test to load a desiredinitial pattern into all the scan paths 104 of circuit 100. After thescan paths have been initialized, the serial mode (SM) signal is set toa second logic level which causes the multiplexers 303 to couple thescan inputs (SI-1-n) 106 of the scan paths to the outputs of formatter306, via scan inputs (SI-1-n) 310, to prepare for testing usingformatter 306. While the serial interface 307 is described as a methodof initializing the scan paths 104 prior to testing using the formatter306, the serial interface 307 can also be used as an alternate method oftesting circuit 100 when the scan paths are placed in the serial modearrangement between SI and SO of bus 307.

FIG. 4 illustrates the formatter circuit 306 in more detail. Theresponse bit streams (RBS1-n) 402 of each response output shift cycle404 from circuit 301 are input to the formatter via SO-1-n. Theformatter responds to the CTL-(1-n) inputs from the tester to format theresponse bit streams into stimulus bit streams (SBS1-n) 406 which areinput to circuit 301 during each stimulus input shift cycle 408. Ifdesired the CTL-(1-n) inputs from the tester can be registered by theSCK prior to being input to the formatter circuit 306, as seen in dottedline area 410.

FIGS. 5A-5D illustrate, according to the disclosure, the operation ofeach section 502-508 of formatter circuit 306 in producing stimulus bitstreams (SBS1-n) from response bit streams (RBS1-n).

FIG. 5A illustrates formatter section 502 providing SBS1 inputs 406 forthe SI-1 input of circuit 301 from RBS1-n outputs 402 of circuit 301during a shift cycle. As the RBS1-n outputs are input to formattersection 502, the CTL-1 input from tester 300 switches to select certainbit segments 510 (A-L) that naturally occur in the RBS1-n outputs 402 ofa “good” circuit 301 to provide the needed SBS1 to the SI-1 input ofcircuit 301. The need for stimulus input on the SI-1 input of circuit301 is hereby, and according to the disclosure, satisfied merely by thereuse of response outputs on SO-1-n of circuit 301.

FIG. 5B illustrates formatter section 504 providing SBS2 inputs 406 forthe SI-2 input of circuit 301 from RBS1-n outputs 402 of circuit 301during a shift cycle. As the RBS1-n outputs are input to formattersection 504, the CTL-2 input from tester 300 switches to select certainbit segments 510 (A-L) that naturally occur in the RBS1-n outputs 402 ofa “good” circuit 301 to provide the needed SBS2 to the SI-2 input ofcircuit 301. The need for stimulus input on the SI-2 input of circuit301 is hereby, and according to the disclosure, satisfied merely by thereuse of response outputs on SO-1-n of circuit 301.

FIG. 5C illustrates formatter section 506 providing SBS3 inputs 406 forthe SI-3 input of circuit 301 from RBS1-n outputs 402 of circuit 301during a shift cycle. As the RBS1-n outputs are input to formattersection 506, the CTL-3 input from tester 300 switches to select certainbit segments 510 (A-L) that naturally occur in the RBS1-n outputs 402 ofa “good” circuit 301 to provide the needed SBS3 to the SI-3 input ofcircuit 301. The need for stimulus input on the SI-3 input of circuit301 is hereby, and according to the disclosure, satisfied merely by thereuse of response outputs on SO-1-n of circuit 301.

FIG. 5D illustrates formatter section 508 providing SBSn inputs 406 forthe SI-n input of circuit 301 from RBS1-n outputs 402 of circuit 301during a shift cycle. As the RBS1-n outputs are input to formattersection 508, the CTL-n input from tester 300 switches to select certainbit segments 510 (A-L) that naturally occur in the RBS1-n outputs 402 ofa “good” circuit 301 to provide the needed SBSn to the SI-n input ofcircuit 301. The need for stimulus input on the SI-n input of circuit301 is hereby, and according to the disclosure, satisfied merely by thereuse of response outputs on SO-1-n of circuit 301.

The bit segments 510 of FIGS. 5A-5D may contain any number of bits. Alsowhile this example shows using 12 bit segments A-L, any number of bitsegments may be used to produce a required stimulus bit stream.

FIG. 6A illustrates that a multiplexer 602 can be used to implement theformatter section 502 of FIG. 5A. The multiplexer has inputs connectedto the SO-1-n outputs of circuit 301, control inputs connected to theCTL-1 input from tester 300, and an output connected to the S1-1 inputof circuit 301. The formatter sections 504-508 of FIGS. 5B-5D cansimilarly be implemented with multiplexers 602. In addition tomultiplexers, other types of switching circuits could be used toimplement formatter sections as well. If desired, and as shown in dottedline, each multiplexer 602 of a formatter may include additional inputsfor a fixed logic one and logic zero. This would allow always being ableto select a logic level that may not appear in any of the response bitstreams (RBS1-n) 402. However with a sufficient number of response bitstreams (RBS1-n) 402 input to the multiplexer 602 it is very likely thata fixed logic level will always be available from one of the SO1-noutputs 108 of circuit 301.

FIG. 6B illustrates times 606 when the CTL-1 input to multiplexer 602 isswitched by the tester 300 to select a response bit segment 510 (A-L) tobe output from multiplexer 602 to form a formatted serial bit stream 604for input to the SI-1 input circuit 301 during a shift cycle. As seen,the tester 300 only needs to switch the CTL-1 input to multiplexer 602,at times 606, whenever it is necessary to select the next response bitsegment 510 to be output to the SI-1 input of circuit 301. As seen, theA segment from RBS1 402 includes a single bit, the B section of RBS3 402includes plural bits, and so on through L which includes plural bits.

FIG. 6C illustrates the same stimulus bit stream 604 being provided onthe SI-1 input of circuit 100 from the stimulus data memory 204 oftester 200 of FIG. 2. As seen, the tester 200 must output a stimulus bitfrom memory 204 during each SCK of a shift cycle, i.e. at times 608, toprovide the stimulus bit stream to SI-1 of circuit 100.

As can be seen in comparing the number of CTL-1 timing outputs 606 fromthe control memory 302 of tester 300 to the number of stimulus bittiming outputs 608 from the stimulus data memory 204 from tester 200 toproduce the stimulus bit stream 604, the control memory 302 of tester300 needs to output a significantly less number of timings than thestimulus data memory 204 of tester 200. Therefore the control memory 302can be much smaller than the stimulus data memory 204. Also since thecontrol timings 606 are output at a lower rate than the stimulus bittimings 608, the control memory 302 does not have to be a high speedmemory, as does stimulus data memory 204. Being able to use a smallerlower speed control memory 302 reduces the cost of tester 300, andtherefore the cost of the product being tested.

FIG. 7 illustrates a formatter circuit 306 providing formatted stimulusdata to “n” scan paths 104. In this and following example Figures, themultiplexers 303 that exist between the formatter 306 outputs 310 andthe scan path 104 inputs 106 are not shown for simplification. As seen,the formatter 306 includes “n” multiplexers 502-508 each having anoutput coupled to one of the “n” scan path inputs and each having inputscoupled to the “n” scan path outputs. Each multiplexer is controlled bya separate “m” signal wide control bus (CTL1-n) from tester 300. Havingseparate control buses for each multiplexer allows the tester to operateeach multiplexer independently to produce formatted stimulus input toits associated scan path. However, and as can be seen, a large number of“m” signal wide control buses are required to be input to the formatter306 from the tester 300 to allow selecting each of the “n” scan pathoutputs. For example, if 16 scan paths are used in the example of FIG.7, the control bus input to each multiplexer would need to be 4 signalswide to allow for selecting any of the serial outputs (SO-1-16) of the16 scan paths, which would require 64 control signals from tester 300.

It should be understood that while the example of FIG. 7 shows all “n”scan path outputs being coupled to multiplexers 502-508, that need notalways be the case. Indeed, it may be possible to produce stimulus datato a scan path using a fewer number of scan path outputs. For example,if the stimulus input to scan path 1 can be satisfied by using only thescan outputs of scan paths 1, 3, 5 and n, the number of control inputsto multiplexer 502 can be reduced down to 2. Likewise, if the stimulusinput to the other scan paths 2-n can be similarly satisfied by usingonly four of the scan path outputs, the number of control inputs fromtester 300 can be significantly reduced.

FIG. 8 illustrates an example of how a circuit 802 can be used as aninterface within a device under test, according to the disclosure, toallow reducing the number multiplexer 502-508 control inputs (CTL1-n)between a tester 803 and formatter 306. Circuit 802 comprises a writestrobe sequencer 804, a first series of m-bit wide registers (R1-Rn)806, and a second series of m-bit wide registers (R1-Rn) 808. The writestrobe sequencer 804 receives a write strobe (WR) input and the SCKinput from tester 803 and outputs separate write strobes (WR1-WRn) toeach of the registers R1-Rn 806. Each R1-Rn register 806 has inputs forinputting data from an m-bit wide data bus from tester 803 and outputsfor outputting m-bit wide data to a corresponding m-bit wide registerR1-Rn 808. Each R1-Rn register 808 inputs the m-bit wide data from itscorresponding R1-Rn register 806 and the SCK signal and outputs m-bitwide data to the control inputs (CTL1-n) of formatter multiplexers502-508.

When WR strobes are input from tester 803, the write strobe generatorsequences its WR1-WRn outputs to cause R1-Rn registers 806 tosequentially load data from tester 803 via the m-bit wide data bus. Whendata has been loaded into each of R1-Rn registers 806, the testeroutputs an SCK to cause the data in R1-Rn registers 806 to betransferred into R1-Rn registers 808 to be applied to the formattercontrol inputs (CTL1-n). Also in response to SCK the write strobesequencer 804 is initialized to prepare it for a subsequent WR strobeinput sequence from the tester.

FIG. 8A illustrates one example implementation of the write strobesequencer 804. As seen the write strobe sequencer 804 includes acircular shift register comprising flips flops (FF) 1-n, a start bit (S)FF, and gating (G) circuitry. In response to an SCK input FFs 1-n areset to a logic low and FF S is set to a logic high start bit. When WRstrobes occur the logic high start bit in FF S is circulated througheach FF 1-n. Each time a FF 1-n receives the logic high start bit itsassociated gate (G) is enabled to pass a WR strobe signal from thetester to an associated R1-Rn register 806 via write strobes WR1-WRn.This process of initializing the circular shift register thencirculating a start bit through it to produce write strobes WR1-WRn isrepeated between each occurrence of the SCK signal.

FIG. 9 illustrates another example of how a circuit 902 can be used asan interface within a device under test, according to the disclosure, toallow reducing the number multiplexer 502-508 control inputs (CTL1-n)between a tester 903 and formatter 306. Circuit 902 comprises a writestrobe addressing circuit 904, a first series of m-bit wide registers(R1-Rn) 806, and a second series of m-bit wide registers (R1-Rn) 808.The write strobe addressing circuit 904 receives a write strobe (WR)input and address (ADD) inputs from tester 903 and outputs separatewrite strobes (WR1-WRn) to each of the registers R1-Rn 806. Each R1-Rnregister 806 has inputs for inputting data from an m-bit wide data busfrom tester 903 and outputs for outputting m-bit wide data to acorresponding m-bit wide register R1-Rn 808. Each R1-Rn register 808inputs the m-bit wide data from its corresponding R1-Rn register 806 andthe SCK signal and outputs m-bit wide data to the control inputs(CTL1-n) of formatter multiplexers 502-508.

When a WR strobe is input from tester 803, the address (ADD) input towrite strobe addressing circuit 904 directs the write strobe to one ofthe write strobe outputs WR1-WRn which causes an associated R1-Rnregister 806 to load data from tester 903 via the m-bit wide data bus.When data has been loaded into one or more of the R1-Rn registers 806,the tester outputs an SCK to cause the data in R1-Rn registers 806 to betransferred into R1-Rn registers 808 to be applied to the formattercontrol inputs (CTL1-n). As can be seen, circuit 902 differs fromcircuit 802 in that only addressed registers R1-Rn 804 are loaded withdata between SCK occurrences as opposed to sequentially loading allregisters R1-Rn 804 with data between SCK occurrences. This isadvantageous since not all registers R1-Rn 806 may need to be loadedwith data between occurrences of SCKS.

FIG. 9A illustrates one example implementation of the write strobeaddressing circuit 904. As seen the write strobe addressing circuit 904includes an address decoder and gating (G) circuitry. In response to theaddress (ADD) input the decoder enables one of the gating (G) circuitsto pass a WR strobe signal from the tester to an associated R1-Rnregister 806 via write strobes WR1-WRn. This process of inputting anaddress then passing a write strobe to an addressed register R1-Rn 806is repeated between each occurrence of the SCK signal.

While FIGS. 8, 8A, 9, and 9A have illustrated ways to reduce the widthof the control (CTL1-n) inputs to formatter 306, other ways may be used.For example, well known high speed SERDES (Serialize/De-serialize)interfaces could be used, as well as others such as high speed DDR(Double Date Rate) interfaces.

FIG. 10 illustrates a formatter circuit 306 providing formatted stimulusdata to “n” scan paths 104. As seen, the formatter includes “n”multiplexers 502-508 each having an output coupled to one of the “n”scan path inputs and each having inputs coupled to the “n” scan pathoutputs. Each multiplexer is controlled by a common “m” signal widecontrol bus (CTL) from a tester. Using a common “m” signal wide controlbus for each multiplexer significantly reduces the number of controlinputs from the tester as compared to the formatter circuit arrangementof FIG. 7. For example, if 256 scan paths are used in the example ofFIG. 10, the common “m” signal wide control bus to each multiplexer502-508 would need to only be 8 signals wide to allow for selecting anyof the serial outputs (SO-1-256) of the 256 scan paths to be used toprovide stimulus inputs to the 256 scan paths 104.

While the example of FIG. 10 shows all “n” scan path outputs beingcoupled to multiplexers 502-508, that need not always be the case.Indeed, it may be possible to produce stimulus data to scan paths 1-nusing a fewer number of scan path serial outputs. For example, if thestimulus inputs to scan paths 1-n 104 can be satisfied by using only thescan outputs of a subset of the scan paths 1-n, the number of signalinputs on the common control bus can be reduced. For example, if 256scan paths are used but only the serial outputs of 16 of the 256 scanpaths are required to produce stimulus inputs to the 256 scan paths, thecommon control bus would require only 4 control inputs from a tester,instead of 8 control inputs.

Using a common control bus (CTL) as shown in FIG. 10 means that allmultiplexer 502-508 control inputs to formatter 306 will be timedtogether. Also the control input to formatter 306 may occur for one ormore SCK occurrences. The following describes one way of providingformatted stimulus inputs to scan paths when using a common formatter306 control bus input.

FIG. 11 illustrates the results of running a conventional automatic testpattern generation (ATPG) software tool, which is provided by variousdesign automation companies, on a circuit 100 within a circuit 301 toproduce a stimulus input pattern set 1102 that will provide a desiredresponse output pattern set 1104 from the circuit 100 within circuit 301to be scan tested. The stimulus input pattern set 1102 comprisesstimulus inputs that will be input to circuit 100 within circuit 301during each shift cycle 1106. Each stimulus input 1106 comprisesstimulus bit streams 1-n (SBS1-n) 1110 for each serial input (SI1-n) tothe scan paths 1-n 104 of circuit 100. The response output pattern set1104 comprises response outputs that will be output from circuit 100within circuit 301 during each shift cycle 1108. Each response output1108 comprises response bit streams 1-n (RBS1-n) 1112 that are expectedto be output on the serial outputs (SO1-n) of each scan path 1-n 104 ofcircuit 100 within circuit 301.

As will be described in FIG. 12, this disclosure describes a process foranalyzing the response output pattern set 1104 of circuit 301 to producea stimulus input pattern set 1102 to circuit 301 via a formatter circuit306 that uses the common control bus (CTL) approach described in regardto FIG. 10. The process involves the steps of analyzing each responsebit stream 1112 of each response output 1108 to find response bit streamsegments that can be formatted, via a formatter 306, and used asstimulus inputs 1106 to circuit 301.

FIG. 12 illustrates the process of analyzing the response bit streams1-n (RBS1-n) 1112 of a response output 1108 of a circuit 301 during ashift cycle 1108 to provide stimulus bit streams 1-n (SBS1-n) 1110 thatcan be used to provide a stimulus input 1106 to circuit 301 during ashift cycle 1106. As seen, common formatter control timings 1-m from thecontrol bus (CTL) of FIG. 10 are established that partition the RBS1-ninto segments 1202. During each control timing 1-m, the segments 1202 ofeach RBS1-n output are analyzed to find response bit segments 1202 thatcan be used as SBS1-n inputs 1112 to circuit 301. For example, the SBS1input to circuit 301 is shown in column 1204 being formed by using bitsegment 1 1202 from RBS1, bit segment 2 1202 from RBS4, bit segment 31202 from RBSn−3, bit segment 4 1202 from RBSn, bit segment 5 1202 fromRBS5, bit segment 6 1202 from RBSn−2, bit segment 7 1202 from RBSn−1,bit segment 8 1202 from RBS3, and on to bit segment “m” 1202 from RBS2.Similarly the SBS2-n inputs to circuit 301 are shown in columns1206-1210 being similarly formed by using bit segments 1202 of theRBS1-n outputs.

Any number of control timings 1-m can be used. Also each of the controltimings 1-m can provide bit segments 1202 that contain one or moreresponse bits. Further, the response bit segments 1202 may be selectedfrom all the RBS1-n outputs or from only a subset of the RBS1-n outputs.The process described above is repeated on the RBS1-n 1112 of eachresponse output 1108 of circuit 301 to produce the desired SBS1-n 1110for each stimulus input 1106 to circuit 301.

It should be understood at this point and going forward that as thenumber of shorter length parallel scan paths increases, a greateropportunity exists to “mine for” response bits that can be used asstimulus bits and, as will be described later, expected data bits. Thusthe efficiency of the disclosure increases as the number of parallelscan paths increases. This naturally follows a trend in scan testingtoday whereby the use of a larger number of shorter length parallel scanpath arrangements is extremely desirable, since that leads to asignificant reduction in test time due to shorter shift cycle times.

FIG. 13 is provided to illustrate that the control input 1306, in dottedline, from a tester 1302 to a formatter 306 of a circuit 1304 can beachieved using any of the methods described and shown in regard to FIGS.3, 7, 8, 9 and 10, i.e. by using the separate control buses (CTL1-n) ofFIGS. 3 and 7, by using circuit 802 of FIG. 8, by using circuit 902 ofFIG. 9, or by using the common control bus (CTL) of FIG. 10.

FIG. 14 illustrates the formatter circuit 306 being used in combinationwith a compactor circuit 1406, according to the disclosure. Compactorcircuits 1406 serve to compact a large number of scan outputs (SO1-n)from a circuit 301 down to a smaller number of compacted data outputs(CDO1-n) 1408 which are output to a tester 1402. Compactor circuits 1406are well known in the test industry and are typically realized usingexclusive OR (XOR) gate trees that input a large number of scan outputsfrom a circuit 301, XOR the scan outputs together, and output aresulting one or more compacted data outputs (CDO1-n) 1408 to a tester.

FIG. 15 illustrates the formatter circuit 306 being used in combinationwith a multiple input signature register (MISR) circuit 1506, accordingto the disclosure. MISR circuits 1506 serve to compress the scan outputs(SO1-n) from a circuit 301 into a signature in response to SEN and SCKcontrol inputs from the tester. At the end of test, the signature issent to the tester 1502 via a serial bus 1508 which may be a JTAG serialbus. MISR circuits 1506 are well known in the test industry and aretypically realized using linear feedback shift registers which input thescan outputs and compress them into a signature. The tester's controlcircuit 210 inputs the signature after the test completes, via bus 1508of bus 212, and determines whether it matches an expected signature.

FIG. 16 illustrates another aspect of the disclosure whereby a secondformatter circuit 306 is used in a device 1604 to provide expected data(ED1-n) inputs to a compare pass/fail circuit 208 which is also used inthe device 1604. Incorporating the expected data formatter 306 andcompare pass/fail circuit 208 into the device reduces the complexity ofthe tester 1602 since it eliminates the need of the tester 1602 to havea compare pass/fail circuit 208 and expected data memory 206 as shown inFIG. 3. Also as can be seen, the number of interconnects between thetester 1602 and the device 1604 is reduced to only needing a CTL bus1306, the SEN 110 and SCK 112 signals, the serial interface 307, and apass/fail bus 1606, which may be a JTAG bus that already exists on thedevice 1604. The expected data formatter 306 is controlled by the tester1602 via a control bus 1306. The control bus 1306 to the expected dataformatter 306 may be the same control bus 1306 that controls thestimulus data formatter 306 or it may be a separate control bus 1306.

FIG. 17 illustrates the expected data formatter circuit 306 in moredetail. The response bit streams (RBS1-n) 402 of each response outputshift cycle 404 from circuit 301 are input to the expected dataformatter via SO-1-n. The expected data formatter responds to theCTL-(1-n) inputs 1306 from the tester to format the response bit streamsinto expected data bit streams (EBS1-n) 1702 which are input to thecompare pass/fail circuit 208 during each shift cycle 408. If desiredthe CTL-(1-n) inputs 1306 from the tester can be registered by the SCKprior to being input to the expected data formatter 306, as seen indotted line area 1706. As can be seen, the operation of the expecteddata formatter 306 is similar to the operation of the stimulus dataformatter 306 of FIG. 4.

FIGS. 18A-18D illustrate, according to the disclosure, the operation ofeach section 502-508 of expected data formatter circuit 306 in producingexpected data bit streams (EBS1-n) from response bit streams (RBS1-n) inresponse to CTL1-n inputs 1306 from the tester. The structure andoperation of the expected data formatter 306 sections 502-508 of FIGS.18a -18D are the same as the stimulus data formatter 306 sections502-508 of FIG. 5a -5D. The only difference between FIGS. 5A-5D andFIGS. 18A-18D is that an expected data bit stream may not be producedusing a response data bit stream that is to be compared against theproduced expected data bit stream. For example, formatter section 502 ofFIG. 18A cannot use the RBS1 output from circuit 301 to produce the EBS1input to compare pass/fail circuit 208. Similarly, formatter section 504cannot use the RBS2 output of circuit 301 to produce the EBS2 input tocompare pass/fail circuit 208, and so on. The reason for this is obvioussince if a failing response output signal were used to provide anexpected data signal to compare against the failing response outputsignal, no failure would be detected since the failing response outputsignal would be compared against itself.

FIG. 19 illustrates an expected data formatter circuit 306 providingformatted expected data (ED1-n) to the compare pass/fail memory 208using separate control buses (CTL1-n) 1306. The structure and operationof the expected data formatter circuit 306 of FIG. 19 is the same as thestimulus data formatter circuit 306 of FIG. 7 with the exception thatthe expected data formatter circuit provides expected data inputs(ED1-n) to the compare pass/fail circuit 208 instead of stimulus data tothe scan paths 104 of circuit 301. The separate control inputs (CTL1-n)1306 to each formatter section 502-508 may be provided directly by thetester as described in FIG. 7, provided using a circuit 802 as describedin FIG. 8, or provided using a 902 as described in FIG. 9.

FIG. 20 illustrates an expected data formatter circuit 306 providingformatted expected data (ED1-n) to the compare pass/fail memory 208using a common control bus (CTL) 1306. The structure and operation ofthe expected data formatter circuit 306 of FIG. 20 is the same as thestimulus data formatter circuit 306 of FIG. 10 with the exception thatthe expected data formatter circuit provides expected data inputs(ED1-n) to the compare pass/fail circuit 208 instead of stimulus data tothe scan paths 104 of circuit 301. The common control bus (CTL) 1306 toeach formatter section 502-508 is provided directly by the tester asdescribed in FIG. 10.

As will be described in FIG. 21, this disclosure describes a process foranalyzing the response output pattern set 1104 of circuit 301 to producean expected data input pattern set 1704 for input to a compare pass/failcircuit 208, via a formatter circuit 306 that uses the common controlbus (CTL) 1306 approach described in regard to FIG. 20 above. Theprocess involves the steps of analyzing each response bit stream 1112 ofeach response output 1108 to find response bit stream segments that canbe formatted, via a formatter 306, and used as expected data inputs 1704to the compare pass/fail circuit 208.

FIG. 21 illustrates the process of analyzing the response bit streams1-n (RBS1-n) 1112 of a response output 1108 of a circuit 301 during ashift cycle to provide expected data bit streams 1-n (EBS1-n) 1702 thatcan be used to provide expected data input to a compare pass/failcircuit 208 during a shift cycle. As seen, common formatter controltimings 1-m from the control bus (CTL) 1306 of FIG. 20 are establishedthat partition the RBS1-n into segments 2102. During each control timing1-m, the segments 2102 of each RBS1-n output are analyzed to findresponse bit segments 2102 that can be used as EBS1-n inputs 1702 tocompare pass/fail circuit 208. For example, the EBS1 input to comparepass/fail circuit 204 is shown in column 2104 being formed by using bitsegment 1 2102 from RBS2, bit segment 2 2102 from RBS4, bit segment 32102 from RBSn−3, bit segment 4 2102 from RBSn, bit segment 5 2102 fromRBS5, bit segment 6 2102 from RBSn−2, bit segment 7 2102 from RBSn−1,bit segment 8 2102 from RBS3, and on to bit segment “m” 2102 from RBS2.Similarly the SBS2-n inputs to compare pass/fail circuit 208 are shownin columns 2106-2110 being similarly formed by using bit segments 2102of the RBS1-n outputs. As previously mentioned in regard to FIGS.18A-18D and as indicated in shaded areas 2112 of FIG. 21, certain RBS1-ninputs to formatter 306 cannot be used to produce EBS1-n outputs fromformatter 306.

Any number of control timings 1-m can be used. Also each of the controltimings 1-m can provide bit segments 2102 that contain one or moreresponse bits. Further, the response bit segments 2102 may be selectedfrom all the RBS1-n outputs, except as mentioned above in regard toFIGS. 18A-18D, or from only a subset of the RBS1-n outputs. The processdescribed above is repeated on the RBS1-n 1112 of each response output1108 of circuit 301 to produce the desired EBS1-n 1702 inputs for eachexpected data input 1704 to compare pass/fail circuit 208.

FIG. 22 illustrates a circuit 100 to be tested using “n” parallel scanpaths (SP1-n) 104. A tester testing circuit 100 would need to interfacewith the circuit using “n” scan inputs (SI1-n) 106 and “n” scan outputs(SO1-n) 108, as well as the SEN 110 and SCK 112 signals. If the circuit100 had 128 scan paths, a tester would need interfacing for 128 scaninputs, 128 scan outputs, and the SEN and SCK inputs (i.e. 258 signals).Using a large number of shorter length scan paths to test circuit 100 isbeneficial since it takes less time to perform shift cycle operations,which reduces test time. However as seen the number of interfaces to atester increases as the number of scan paths increases.

FIG. 23 illustrates an arrangement 2301 whereby the circuit 100 of FIG.22 is interfaced to the stimulus data formatter 306 of FIG. 16 to driveits scan inputs 106 and a circuit 2302, comprising the expected dataformatter 306 and compare pass/fail circuit 208 of FIG. 16, to receiveits scan outputs 108. As described in FIG. 3A, multiplexers 303 are alsoprovided at the scan inputs 106 of circuit 100 to provide forinitializing the scan paths 104 of circuit 100 via the serial interface307. As seen, the interface to a tester would include the serialinterface 307, control bus (CTL) 306, the pass/fail output bus 1606 andthe SEN 110 and SCK 112 control signals. If the circuit 100 had 128 scanpaths 104, as mentioned in FIG. 22, and used a common control bus (CTL)as shown in FIGS. 10, 16 and 20, the tester would only need interfacingto provide the 3 serial interface signals 307 to initialize the scanpaths 104, 7 or less common control bus (CTL) signals 1306 to operatethe stimulus and expected data formatters 306, the pass/fail bus signals1606, which could be a single signal, and the SEN 110 and SCK 112 inputsignals. It should be understood that circuit 2302 could also be theMISR circuit 1506 of FIG. 15 and the pass/fail bus could be the serialbus 1508 of FIG. 15 that unloads the test signature at the end of test.

In comparing the tester interface signal requirements of FIGS. 22 and23, it is clear that the disclosure provides an approach thatsignificantly decreases the number of interface signals between a testerand a circuit to be tested that uses a large number of scan paths, whichdecreases the tester cost. Also, smaller interfaces between a circuit tobe tested and a tester enables more circuits to be tested in parallel.

FIG. 24 illustrates an arrangement 2401 whereby the circuit arrangement2301 of FIG. 23 is augmented with gating circuits 2404-2409 on the CTL1306, SCK 112, SEN 110, pass/fail bus 1606, and SO 307 signals, and anenable signal 2410 to control the gating circuits. In response to theenable signal 2410 the CTL, SCK and SEN inputs to the circuit 2402 andthe pass/fail bus and SO outputs from the circuit 2402 can be enabled ordisabled. The gating circuit 2404 on the CTL signals 1306 is optional,as indicated in dotted line, and is shown merely as a way to eliminatethe switching operation of the formatter 306 when the enable signal 2410is in the disable state. Disabling the CTL signals 1306 to formatter 306reduces test power consumption and noise when another circuit 2402 isenabled for testing using the CTL signals 1306. The advantage of theenable and gating circuits are described in FIG. 25.

FIG. 25 illustrates an IC 2502 that includes a number of circuitarrangements 2402, each circuit arrangement 2402 having a unique enablesignal 2410. When one of the circuit arrangements 2402 is to be tested,its enable signal is set to allow the CTL, SEN, SCK, pass/fail and SOsignals to pass through the gating circuits 2404-2409. Likewise othercircuit arrangements 2402 are tested by enabling their CTL, SEN, SCK,pass/fail and SO signals. The advantage with separately enable-ablecircuits 2402 is that they can all be interfaced to a common cm, SEN,SCK, pass/fail, and SO signal interface to a tester.

FIG. 26 illustrates an IC 2602 that comprises a JTAG port 2606, anembedded test controller 2604, a control memory 302, stimulus andexpected data formatters 306, compare pass/fail circuit 208, and pluralcircuits 301 to be tested. The JTAG port is a well known IC testinterface defined by IEEE standard 1149.1 that is used to control testcircuitry within the IC. The JTAG port 2606 is connected externally to atester 2608, via TDI, TCK, TMS and TDO signals, and internally to theembedded test controller 2604, compare pass/fail circuit 208 and thecontrol memory 302 via bus 2612. The embedded test controller 2604 issimilar to test controller 210 used in tester 300 with the exceptionthat it is optimized for use within the IC to control the control memory302, the compare pass/fail circuit 208 and the circuit 301 being testedvia control bus 2610.

As seen, the embedded test controller outputs control via bus 2610 to;(1) operate the control memory to output control (CTL) 1306 to thestimulus and expected data formatters 306, (2) operate the comparepass/fail circuit 208, via the SEN and SCK signals, to compare theresponse outputs to expected data inputs, (3) to operate the SEN and SCKsignals to control the selected circuit 301 being tested, (4) to operateenable (ENA) signals 22614 to circuits 301 to allow the SEN and SCKsignals to be input to the selected circuit 301 being tested and theserial outputs (SO-1-n) to be output from the circuit 302 being tested,and (5) to operate the serial interface 307 to initialize the scan paths104 of the circuit 301 being tested. When the enable (ENA) signal to acircuit 301 is active, the SEN and SCK inputs are enabled to control thecircuit's scan paths 104 and the circuit's scan path outputs are enabledto output response data. When the enable (ENA) signal to a circuit 301is inactive, the SEN and SCK inputs are disabled from controlling thecircuit's scan paths 104 and the circuit's scan path outputs are disablefrom outputting response data.

Prior to testing, the tester 2608 operates the JTAG port 2606 to uploadformatter control (CTL) into the control memory 302, via bus 2612, fortesting a particular one of the circuits 301. After the control memoryis loaded, the tester inputs a command, via bus 2612, to the embeddedtest controller 2604 to identify which circuit 301 is to be tested. Inresponse to the command input, the embedded test controller 2604executes an embedded program that initializes the scan paths 104 of theselected circuit 301 via serial interface 307, then tests the selectedcircuit 301 by outputting control (CTL) on bus 2610 to the controlmemory 302, outputting SEN and SCK control to the compare pass/failcircuit 208, and outputting SEN and SCK control to the circuit 301 beingtested.

The tester 2608 monitors the status of the test by polling the embeddedtest controller 2604 via bus 2612 of the JTAG port 2606. When the tester2608 determines, via the polling, that the test is complete, it operatesthe JTAG port 2606 to access the information stored in the comparepass/fail circuit 208 using bus 2612 to determine whether the enabledcircuit 301 has passed or failed the test. Similarly, the other circuits301 of IC 2602 are enabled and tested using the stimulus and responseformatter based test arrangement of FIG. 26.

FIG. 27 illustrates an IC 2702 that comprises the JTAG port 2606 of FIG.26, the embedded test controller 2604 of FIG. 26, the control memory 302of FIG. 26, stimulus data formatter 306 of FIG. 26, the MISR 1506 ofFIG. 15, and plural circuits 301 to be tested. IC 2702 is identical instructure and operation to the IC 2602 of FIG. 26 with the exceptionthat the MISR 1506 is used instead of compare pass/fail circuit 208 andexpected data formatter 306. During test, the embedded controlleroperates the MISR, via the SEN 110 and SCK 112 signals, to compress theserial outputs (SO-1-n) of the selected circuit 301 into a signature.

Prior to testing, the tester 2608 operates the JTAG port 2606 to uploadformatter control (CTL) into the control memory 302, via bus 2612, fortesting a particular one of the circuits 301. After the control memoryis loaded, the tester inputs a command, via bus 2612, to the embeddedtest controller 2604 to identify which circuit 301 is to be tested. Inresponse to the command input, the embedded test controller 2604executes an embedded program that initializes the scan paths 104 of theselected circuit 301 via serial interface 307, then tests the selectedcircuit 301 by outputting control (CTL) on bus 2610 to the controlmemory 302, outputting SEN and SCK control to the MISR 1506, andoutputting SEN and SCK control to the circuit 301 being tested.

The tester 2608 monitors the status of the test by polling the embeddedtest controller via bus 2612 of the JTAG port 2606. When the tester 2608determines, via the polling, that the test is complete, it operates theJTAG port 2606 to access the signature stored in the MISR 1506 using bus2612 to determine whether the enabled circuit 100 has passed or failedthe test. Similarly, the other circuits 100 of IC 2602 are enabled andtested using the formatter and MISR based test arrangement of FIG. 26.

It should be understood that the control memory 302 of FIGS. 26 and 27may be a non-volatile memory (ROM, EPROM) that contains all theformatter control (CTL) 306 patterns required for testing each circuit301. In that case, the uploading step mentioned above in regard to FIGS.26 and 27 would not be necessary and the testing could commence bysimply inputting a command to the embedded test controller 2604 via bus2612 of JTAG port 2606 to execute a program that initializes the scanpaths of the selected circuit 301 then tests the selected circuit 301.

As seen in dotted line, the JTAG port may be used to initialize the scanpaths of a circuit 301, via bus 2612, instead of using the embeddedcontroller's bus 2610 to initialize the scan paths of a circuit 301. Inthis case, the testing could commence by simply inputting a command tothe embedded test controller 2604 via bus 2612 of JTAG port 2606 toimmediately start the execution of a program that tests the selectedcircuit 301.

As can be seen, the advantages of the embedded test architectures ofFIGS. 26 and 27 is that testing of circuits 301 within an IC can beeffectively enabled and executed, according to the disclosure, usingonly a JTAG port on the IC. Since the testing only requires the JTAGport, the testing can be used in a myriad of environments, including ICmanufacture, IC burn-in, IC customer integration and test, and fieldservice testing.

Although the disclosure has been described in detail, it should beunderstood that various changes, substitutions and alterations may bemade without departing from the spirit and scope of the disclosure asdefined by the appended claims.

What is claimed is:
 1. Interface circuitry comprising: (a) a control busof m inputs, a write strobe interface input, and a scan clock interfaceinput; (b) write strobe sequencer circuitry having a write strobesequencer input coupled to the write strobe interface input, a scanclock sequencer input coupled to the scan clock interface input, andwrite strobe interface outputs, the write strobe sequencer including:(i) a circular shift register formed of flips flops, each flip flophaving a write strobe register input coupled to the write strobeinterface input, a scan clock register input coupled to the scan clockinterface input, a circular input, and an output, the flip flops beingconnected in series with the output of a last flip flop being connectedto the circular input of a first flip flop, upon receiving a scan clockregister input the outputs of all of the flip flops except for the lastflip flop being set to a logic low state and the output of the last flipflop being set to a logic high state to form a start bit, upon receivinga write strobe register input the flip flops circulate the start bitthrough the flip flop outputs; and (ii) logic gates, each logic gatehaving an input connected to an output of a flip flop, a write strobeinput coupled to the write strobe interface input, and a gated writestrobe output; (c) a set of first registers, each first register havingm control inputs coupled to the control bus of m inputs, a write strobeinput coupled to a gated write strobe output, and a set of controloutputs; and (d) a set of second registers, each second register havingcontrol inputs coupled to the control outputs of a first register, aclock input coupled to the scan clock interface input, and a set ofcontrol outputs, in which m is an integer greater than
 1. 2. Theinterface circuitry of claim 1 including an integrated circuit having:parallel scan paths with stimulus inputs and stimulus outputs; andformatter multiplexer circuitry, each multiplexer circuit havingstimulus inputs coupled to the stimulus outputs, stimulus outputscoupled to the stimulus inputs, and control inputs coupled to a set ofcontrol outputs of a second register.
 3. The interface circuitry ofclaim 1 including an integrated circuit having: parallel scan paths,each scan path having stimulus inputs and stimulus outputs; andformatter multiplexer circuitry, each multiplexer circuit havingstimulus inputs coupled to the stimulus outputs of one scan path,stimulus outputs coupled to the stimulus inputs of one scan path, andcontrol inputs coupled to a set of control outputs of one secondregister.
 4. The interface circuitry of claim 1 including an integratedcircuit having: functional circuitry having functional stimulus inputsand functional response outputs; parallel scan paths with functionalstimulus outputs coupled to the functional stimulus inputs, functionalresponse inputs coupled to the functional response outputs, scan pathstimulus inputs and scan path stimulus outputs; and formattermultiplexer circuitry, each multiplexer circuit having stimulus inputscoupled to the scan path stimulus outputs, stimulus outputs coupled tothe scan path stimulus inputs, and control inputs coupled to a set ofcontrol outputs of a second register.